1. Field of the Invention
The subject invention generally pertains to electronic power conversion circuits, and more specifically to high frequency, switched mode power electronic converter circuits.
2. Description of Related Art
One significant source of power losses in high frequency power converters is gate drive loss. Some converters have the inherent ability to provide synchronous rectifier self gate drive which results in the recirculation of gate drive energy and easy synchronous rectifier gate drive. Without a self gate drive mechanism driver circuits are required and, in many cases, these driver circuits can be complex, costly, and inefficient. Also, the signals available and easily accessible often do not provide the proper timing or signal levels for synchronous rectifier gate drive.
In most cases a positive voltage is applied to the gate of an N channel power mosfet during the on state of the switch. The positive voltage should be sufficient to fully enhance the switch, but no more. Often a negative gate voltage is applied to the gate during the off state of the switch. The negative gate drive speeds up the turn off transition by increasing the current out of the gate during the transition, which serves to reduce turn off transition losses in the drain circuit.
U.S. patent application Ser. No. 10/157,101 revealed a gate drive mechanism for a synchronous rectifier, illustrated in FIG. 1, that relies on a signal from a small choke added to the power converter to provide energy for a zero voltage turn on transition of a main switch. The placement of transistors and diodes between the small choke and the gate of the synchronous rectifier served to provide optimal timing of the synchronous rectifier during the turn on transition so that the synchronous rectifier is turned on precisely as its drain to source voltage reaches zero volts. The wave forms for the FIG. 1 circuit are illustrated in FIG. 2. There is a mechanism that exists in the normal operation of the circuit that can inadvertently turn on the synchronous rectifier as its drain to source voltage rises at the beginning of the off state of the synchronous rectifier. The problem is illustrated in FIGS. 3(a) and 3(b). As the drain to source voltage of the synchronous rectifier rises there is a current that flows in the intrinsic gate drain capacitance of the mosfet. This current serves to charge the gate to source capacitance of the power mosfet, as illustrated in FIG. 3(b), which, if the gate to source voltage rises above the gate threshold voltage, will turn on the synchronous rectifier during its turn off transition, which may lead to catastrophic results. What is needed is a circuit mechanism that avoids this inadvertent turn of the synchronous rectifier and alternate methods for synchronous rectifier self gate drive that provide improvements to the operation of the power converter as a whole.